1. Field of the Invention
This invention relates to a method and apparatus for igniting and detecting a flame in a burner system using a combustable hydrocarbon fuel.
Burner systems of the type forming the environment for the present invention generally include a main burner connected to a supply of combustable hydrocarbon fuel which is ignited by a pilot light. Various control circuits are known in the art for automatically discontinuing fuel flow to the burner if the flame is extinguished or, alternatively, for automatically reigniting the flame.
2. Description of the Prior Art
Flame detection systems using flame conductivity, per se, are known in the art. Flame conductivity, is expressed by the equation ##EQU1## where n.sub.e .tbd.electron density
e.tbd.electron charge PA0 m.sub.e .tbd.electron mass PA0 .nu.en.tbd.electron-neutral collision frequency
The existance of .sigma. is due to the presence of electrons (of density n.sub.e) which are produced by thermal ionization, according to the well known Saha equation. For typical flame temperatures of 1700.degree. C., n.sub.e is approximately 10.sup.6 electrons/cm.sup.3 at atmospheric pressure, while .nu.en is approximately 10.sup.10 to 10.sup.12.
Examples of prior art flame detection systems which use flame conductivity may be found in U.S. Pat. Nos. 3,941,553, 3,627,458, and 3,836,316.
In these systems, the flame is disposed between a pair of electrodes, one of which is typically the burner itself and by its conductivity completes an electrical circuit therebetween. Such circuits are characterized by impedances of approximately 50 M.OMEGA. and, consequentally, a very small magnitude of current flows through the flame, and must accordingly be considerably amplified prior to use as a controlling signal. It may be readily appreciated that the requirement of high amplification detrimentally effects both the cost and reliability of such systems and that the relatively small current magnitude is undesirable from the standpoint of ambiguous flame identification.
Ignition systems of the prior art generally use spark techniques wherein a spark between an electrode and the burner ignites the combustable fuel in the gap therebetween. These systems require the use of relatively costly and bulky transformers for producing a voltage which is sufficiently high to enable the spark to jump the gap. Additionally, the spark is not a completely reliable ignition source owing to the rapid dissipation of its thermal energy and the relative criticalness of its position within the flame site. Example of such ignition systems may be found in U.S. Pat. Nos. 3,614,280, and 3,766,441.